Digital modulation schemes for use in digital radio communication include digital phase modulation schemes (e.g., BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 8PSK (Phase Shift Keying)), QAM (Quadrature Amplitude Modulation), and the like. When signals modulated by these digital modulation schemes are demodulated, an analog quadrature demodulation section composed of an analog circuit is used to perform quadrature demodulation, and the result is obtained as a complex signal (IQ signal), which is widely practiced.
Here, in an ideal situation, an in-phase component and a quadrature component of the IQ signal output from the analog quadrature demodulation section are orthogonal to each other, and have the same average amplitude. However, the analog quadrature demodulation section is affected by characteristic variations, temperature characteristics, voltage characteristics, and the like of the analog element, so that an amplitude error or a quadrature skew error occurs between the in-phase component and the quadrature component of the IQ signal. Therefore, when the IQ signal output from the analog quadrature demodulation section is directly subjected to detection, a bit error rate (hereinafter referred to as BER) increases.
Patent Document 1 described below discloses a demodulation circuit which determines quadrants on an IQ plane of the IQ signal, stores amplitude values of the IQ signal, depending on the quadrants in which the IQ signal is present, and corrects a quadrature skew error using a difference between the stored amplitude values as quadrature error information.
Patent Document 2 described below discloses an amplitude control circuit which corrects amplitudes of one of an in-phase component and a quadrature component of an input IQ signal using a value obtained by integration of a difference in amplitude value between the in-phase component and the quadrature component using a loop filter.
Patent Document 3 described below discloses a circuit which calculates a coefficient which is adaptively updated by applying the CMA (Constant Modulus Algorithm) to an output of the analog quadrature demodulation section, and corrects and outputs an amplitude error and a quadrature skew error.    Patent Document 1: Japanese Unexamined Patent Publication No. 7-297874    Patent Document 2: Japanese Unexamined Patent Publication No. 2003-258931    Patent Document 3: Japanese Unexamined Patent Publication No. 10-56484